UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                   FORM 10-K

(Mark One)

[X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
ACT OF 1934

         For the fiscal year ended:   December 31, 2005
                                      -----------------

                                      or

[_] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
EXCHANGE ACT OF 1934

         For the transition period from _____________ to _____________

                     Commission file number: 333-123167-03
                                             -------------

                CWALT, Inc., Alternative Loan Trust 2005-21CB,
             Mortgage Pass-Through Certificates, Series 2005-21CB
             ----------------------------------------------------
            (Exact name of Registrant as specified in its charter)


          Delaware                                          87-0698307
-------------------------------                    -----------------------------
(State or other jurisdiction of                          (I.R.S. Employer
incorporation or organization)                           Identification No.)

c/o  Countrywide Home Loans Servicing LP
     4500 Park Granada
     Calabasas, California                                     91302
----------------------------------------           -----------------------------
(Address of principal executive offices)                     (Zip Code)

Registrant's telephone number, including area code:   (818) 225-3000
                                                      --------------

Securities registered pursuant to Section 12(b) of the Act:  None
                                                             ----

Securities registered pursuant to Section 12(g) of the Act:  None
                                                             ----


<PAGE>


     Indicate by check mark if the registrant is a well-known seasoned issuer,
as defined in Rule 405 of the Securities Act. [_] Yes  [X] No

     Indicate by check mark if the registrant is not required to file reports
pursuant to Section 13 or Section 15(d) of the Act. [_] Yes [X] No

     Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to
such filing requirements for the past 90 days. [X] Yes [_] No

     Indicate by check mark if disclosure of delinquent filers pursuant to
Item 405 of Regulation S-K (ss.229.405 of this chapter) is not contained
herein, and will not be contained, to the best of registrant's knowledge, in
definitive proxy or information statements incorporated by reference in Part
III of this Form 10-K or any amendment to this Form 10-K. [X]

     Indicate by check mark whether the registrant is a large accelerated
filer, an accelerated filer, or a non-accelerated filer. See definition of
"accelerated filer and large accelerated filer" in Rule 12b-2 of the Exchange
Act. (Check one):

Large accelerated filer [_]   Accelerated filer [_]   Non-accelerated filer [X]

      Indicate by check mark whether the registrant is a shell company (as
defined in Rule 12b-2 of the Act). [_] Yes [X] No

     State the aggregate market value of the voting and non-voting common
equity held by non-affiliates computed by reference to the price at which the
common equity was last sold, or the average bid and asked price of such common
equity, as of the last business day of the registrant's most recently
completed second fiscal quarter. Not applicable


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<PAGE>


                      DOCUMENTS INCORPORATED BY REFERENCE

                                     None.

                                     * * *


         This Annual Report on Form 10-K (the "Report") is filed with respect
to the trust fund entitled Alternative Loan Trust 2005-21CB (the "Trust Fund")
formed pursuant to the pooling and servicing agreement dated as of April 1,
2005 (the "Pooling and Servicing Agreement") among CWALT, Inc., as depositor
(the "Company"), Countrywide Home Loans, Inc., as a seller (a "Seller"), Park
Granada LLC, as a seller (a "Seller"), Park Sienna LLC, as a seller (a
"Seller"), Park Monaco Inc., as a seller (a "Seller"), Countrywide Home Loans
Servicing LP, as master servicer (the "Master Servicer"), and The Bank of New
York, as trustee (the "Trustee"), for the issuance of CWALT, Inc., Mortgage
Pass-Through Certificates, Series 2005-21CB (the "Certificates"). Certain
information otherwise required to be included in this Report by the
Instructions to Form 10-K has been omitted in reliance on the relief granted
by the Commission to other companies in similar circumstances, in particular,
the relief granted in CWMBS, Inc. (February 3, 1994) (collectively, the
"Relief Letters").


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<PAGE>


                                    PART I


Item 1.           Business.

                  Not applicable.  See the Relief Letters.

Item 1A.          Risk Factors.

                  Not applicable.

Item 1B.          Unresolved Staff Comments.

                  None.

Item 2.           Properties.

                  Not applicable.  See the Relief Letters.

Item 3.           Legal Proceedings.

                  There were no material pending legal proceedings relating to
                  the Trust Fund to which any of the Trust Fund, the Trustee,
                  the Master Servicer or the Company was a party or of which
                  any of their respective properties was the subject during
                  the fiscal year covered by this Report other than ordinary
                  routine litigation incidental to the duties of the Trust
                  Fund, the Trustee, the Master Servicer or the Company, nor
                  is the Company aware of any such proceedings contemplated by
                  governmental authorities.

Item 4.           Submission of Matters to a Vote of Security Holders.

                  No matter was submitted to a vote of Certificateholders, and
                  no Certificateholder consent was solicited during the fiscal
                  year covered by this Report.


                                      4
<PAGE>


                                    PART II


Item 5.           Market for Registrant's Common Equity, Related Stockholder
                  Matters and Issuer Purchases of Equity Securities.

                  (a)   Item 201 of Regulation S-K:

                        i.    Market Information. There is no established
                              public trading market for the Certificates.

                        ii.   Holders. As of December 31, 2005 there were less
                              than 300 holders of record of the Certificates.

                        iii.  Dividends. Not applicable. See the Relief
                              Letters. (Information as to distributions to
                              Certificateholders is provided in the
                              Registrant's monthly filings on Form 8-K.)

                        iv.   Securities Authorized for Issuance Under Equity
                              Compensation Plans. Not applicable.

                        Item 701 of Regulation S-K.  Not applicable.

                  (b)   Rule 463 of the Securities Act of 1933. Not
                        applicable.

                  (c)   Item 703 of Regulation S-K. Not applicable as the
                        Registrant has no class of equity securities
                        registered pursuant to section 12 of the Securities
                        Exchange Act of 1934.

Item 6.           Selected Financial Data.

                  Not applicable.  See the Relief Letters.

Item 7.           Management's Discussion and Analysis of Financial Condition
                  and Results of Operation.

                  Not applicable.  See the Relief Letters.

Item 7A           Quantitative and Qualitative Disclosures about Market Risk.

                  Not applicable.

Item 8.           Financial Statements and Supplementary Data.

                  Not applicable.  See the Relief Letters.


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<PAGE>


Item 9.           Changes in and Disagreements With Accountants on Accounting
                  and Financial Disclosure.

                  None.

Item 9A.          Controls and Procedures.

                  Not applicable.

Item 9B.          Other Information.

                  None.


                                      6
<PAGE>


                                   PART III


Item 10.          Directors and Executive Officers of the Registrant.

                  Not applicable.  See the Relief Letters.

Item 11.          Executive Compensation.

                  Not applicable.  See the Relief Letters.

Item 12.          Security Ownership of Certain Beneficial Owners, Management
                  and Related Stockholder Matters.

                  Information required by Item 201(d) of Regulation S-K: Not
                  applicable.

                  Information required by Item 403 of Regulation S-K:

                  (a)   Not applicable. See the Relief Letters.

                  (b)   Not applicable. See the Relief Letters.

                  (c)   Not applicable. See the Relief Letters.

Item 13.          Certain Relationships and Related Transactions.

                  Not applicable.  See the Relief Letters.

Item 14.          Principal Accounting Fees and Services.

                  Not applicable.


                                      7
<PAGE>


                                    PART IV


Item 15.          Exhibits and Financial Statement Schedules.

                  (a)   (1) Pursuant to the Pooling and Servicing Agreement,
                        the Master Servicer is required (i) to deliver an
                        annual statement as to compliance with the provisions
                        of the Pooling and Servicing Agreement and certain
                        other matters (the "Annual Statement of the Master
                        Servicer") and (ii) to cause a firm of independent
                        public accountants to deliver an annual report as to
                        compliance with the servicing provisions of the
                        Pooling and Servicing Agreement (the "Annual Report of
                        the Firm of Accountants"). The Annual Statement of the
                        Master Servicer is attached as Exhibit 99.1 to this
                        Report. The Annual Report of the Firm of Accountants
                        is attached as Exhibit 99.2 to this Report. See Relief
                        Letters.

                        (2)   Not applicable.

                        (3)   The required exhibits are as follows:

                              o     Exhibit 3(i): Copy of Company's
                                    Certificate of Incorporation (Filed as an
                                    Exhibit to Registration Statement on Form
                                    S-3 (File No. 333-110343)).

                              o     Exhibit 3(ii): Copy of Company's By-laws
                                    (Filed as an Exhibit to Registration
                                    Statement on Form S-3 (File No.
                                    333-110343)).

                              o     Exhibit 4: Pooling and Servicing Agreement
                                    (Filed as part of the Registrant's Current
                                    Report on Form 8-K filed on August 25,
                                    2005).

                              o     Exhibit 31: Rule 15d-14(a) Certification.

                              o     Exhibit 99.1: Annual Statement of the
                                    Master Servicer.

                              o     Exhibit 99.2: Annual Report of the Firm of
                                    Accountants.

                  (b)   See subparagraph (a)(3) above.

                  (c)   Not applicable. See the Relief Letters.


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<PAGE>


                                  SIGNATURES

         Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this Report to be signed
on its behalf by the undersigned, thereunto duly authorized.

                                    CWALT, INC.,
                                    ALTERNATIVE LOAN TRUST 2005-21CB,
                                    MORTGAGE PASS-THROUGH CERTIFICATES,
                                    SERIES 2005-21CB

                                    By: Countrywide Home Loans Servicing LP,
                                        as Master Servicer*

                                    By: Countrywide GP, Inc.


                                    By: /s/ Craig Baingo
                                        ----------------
                                    Name:  Craig Baingo
                                    Title: Executive Vice President

                                    Date:  March 27, 2006


---------
*     This Report is being filed by the Master Servicer on behalf of the Trust
      Fund. The Trust Fund does not have any officers or directors.


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<PAGE>


         SUPPLEMENTAL INFORMATION TO BE FURNISHED WITH REPORTS FILED PURSUANT
TO SECTION 15(d) OF THE ACT BY REGISTRANTS WHICH HAVE NOT REGISTERED
SECURITIES PURSUANT TO SECTION 12 OF THE ACT.

         No such annual report, proxy statement, form of proxy or other
soliciting material has been, or will be, sent to Certificateholders. See the
Relief Letters.


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<PAGE>


                                 EXHIBIT INDEX



Exhibit           Document


3(i)              Company's Certificate of Incorporation (Filed as an Exhibit
                  to Registration Statement on Form S-3 (File No.
                  333-110343))*

3(ii)             Company's By-laws (Filed as an Exhibit to Registration
                  Statement on Form S-3 (File No. 333-110343)*

4                 Pooling and Servicing Agreement (Filed as part of the
                  Company's Current Report on Form 8-K filed on August 25,
                  2005)*

31                Rule 15d-14(a) Certification

99.1              Annual Statement of the Master Servicer

99.2              Annual Report of the Firm of Accountants




---------
*     Incorporated herein by reference.


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